How is a call to instance_methods dispatched in Ruby?

Question

instance_methods is defined as a public instance method within the Module class. Why and how are we then able to call Object.instance_methods, which is the syntax for class method invocation?

Answer 1

Because instance_methods is a instance method on Module, that method can be called on any instance of the Module class or it's subclasses.

As it turns out, Object is an instance of the Class class:

Object.instance_of? Class
#=> true

And, Class is a subclass of Module:

Class < Module
#=> true

Here's a helpful chart illustrating the class hierarchy of the various objects in Ruby. Notice how Module is listed as a superclass of Class, which all Classes in Ruby are instances of:

View full size

Answer 2

There is no such thing as the "syntax of class method invocation". There is also no such thing as a "class method".

This is just method invocation like any other method invocation. You are calling the method instance_methods on the object referenced by the constant Object. That object is the Object class, which is an instance of the Class class. Class is a subclass of Module, and so the Object class is an (indirect) instance of the Module class which defines the instance_methods method.

Answer 3

It looks like a class method in this case, but in Ruby, Object is just an instance of Class, which has Module as a super class. So what looks like a class method here is actually an instance method invoke on instance Object of class Class.

Object.instance_of? Class # => true
Object.is_a? Module #=> true